Gas-fired thermoforming machine

ABSTRACT

To obtain a mixture of air and a combustible gas for burning, a first plurality of elongated infrared radiation gas burners forming a bottom section and a second plurality of elongated parallel infrared radiation gas burners forming a top section of an oven of a thermoforming machine communicate with four manifolds, with each of the burners in a lower section communicating with two manifolds through two different gas lines and with each of the burners in the upper section communicating with the other two manifolds through two different lines, one of the lines between each burner and the manifold including a manually adjustable valve for controlling, within a range, the flow of gas to the burner. Gas is provided to the manifolds through gas lines under the control of (1) venturi type gas mixers; (2) automatically controlled regulating valves; (3) manual valves; and (4) valves that close the gas line between cycles of the thermoforming machine to clear the lines with air, and, when an infrared photocell determines that the plastic sheet is too hot, to cool the plastic sheet by blowing air upon it.

United States Patent Kostur 1 Sept. 19, 1972 [54] GAS-FIRED THERMOFORMING MACHINE [72] Inventor: Robert E. Kostur, Oak Brook, Ill.

[73] Assignee: Comet Industries, Inc., Bensenville,

Ill.

[22] Filed: Dec. 24, 1970 [21] Appl. No.: 101,322

[52] 0.8. CI. ..263/40 R, 263/3 [51] Int. Cl ..F27b 3/00 [58] Field of Search ..263/3, 40 R, 43

[56] References Cited UNITED STATES PATENTS 3,406,954 10/1968 Fannon, Jr ..263/3 3,445,096 5/1969 Seefluth ..263l3 Primary Examiner.lohn J. Camby Attorney-Fidler, Patnaude, Lazo & Batz [57] ABSTRACT To obtain a mixture of air and a combustible gas for burning, a first plurality of elongated infrared radiation gas burners forming a bottom section and a second plurality of elongated parallel infrared radiation gas burners forming a top section of an oven of a therrnoforming machine communicate with four manifolds, with each of the burners in a lower section communicating with two manifolds through two different gas lines and with each of the burners in the upper section communicating with the other two manifolds through two different lines, one of the lines between each burner and the manifold including a manually adjustable valve for controlling, within a range, the flow of gas to the burner. Gas is provided to the manifolds through gas lines under the control of (1) venturi type gas mixers; (2) automatically controlled regulating valves; (3) manual valves; and (4) valves that close the gas line between cycles of the therrnoforming machine to clear the lines with air, and, when an infrared photocell determines that the plastic sheet is too hot, to cool the plastic sheet by blowing air upon it.

10 Claims, 5 Drawing Figures GAS-FIRED THERMOFORMING MACHINE This invention relates to thermoforming apparatuses and more particularly relates to ovens for thermoforming apparatuses.

One type of oven for thermoforming apparatuses includes a first bank of heaters forming an upper section of the oven and a second bank of heaters forming a lower section of the oven. A sheet of plastic is brought between the first and second banks of heaters to be softened prior to forming.

- In the prior art, the heaters in the upper and lower sections of the ovens were generally electrical heaters. Electrical heaters have several disadvantages such as: l) the electrical power they use is relatively expensive; (2) the electrical heaters require a relatively long time to cool, thus slowing the operation of the machine and increasing maintenance by causing clamp frame cylinders to become overheated; (3) if the plastic sheets become overheated, a relatively long time is necessary to cool the sheets; and (4) it is difficult to obtain the proper frequency spectrum of infrared radiation from electrical heaters for the particular plastic to be heated.

It is known to use gas burners to heat plastics for forming but thermoforming machines that are gas fired are not known.

Accordingly, it is an object of the invention to provide a novel oven.

. It is a further object of the invention to provide a novel oven for thermoforming apparatuses.

It is a still further object of the invention to provide an improved control system for gases that are supplied to gas burners.

It is a still further object to provide an oven that is able to cool -an item within the oven when the item became overheated.

It is a still further object of this invention to provide To provide gas to the burners, there are two manifolds for the lower section and two manifolds for an oven for thermoforming apparatus in which each of a plurality of infrared-radiation gas burners receive gases from a plurality of manifolds, at least one of which has a means for adjusting the flow between the manifold and the burner to control the pressure of the gas in the burner and therefore the frequency spectrum of infrared radiation from the burner.

7 It is a still further object of this invention to provide a gas-fired oven for use in thermoforming apparatus in which a means is provided for clearing the lines of combustible gas with air. I

It is a still further object of this invention to provide a gas-fired oven for thermoforming apparatuses which automatically blows air upon a heated sheet of plastic when the plastic becomes too hot. I

In accordance with the above and further objects of the invention, an oven for a thermoforming apparatus is provided having a lower section including a plurality of parallel elongated infrared-radiation gas burners and an upper section including a similar plurality of elongated parallel infrared-radiation gas burners.

Each of the gas burners includes a lower chamber for receiving the combustible gases and an upper chamber with adjustable metering orifices between the chambers. The upper chambers are covered with a small mesh wire so that combustion is complete within the burners and no exhaust stacks are necessary. The burners in the lower section and in the upper section face each other to heat a sheet of plastic between them.

the upper section. Each burner is connected to the two different manifolds through a different line, one of the lines for each burner having a manually adjustable valve or stopcock which is used to control the rate of flow and pressure of the gases to the burner. The burners are in sections, each of which includes an adjustable distributor so that the heat from a burner may be adjusted for different values along its length.

The two lower manifolds receive gases through lines that include a first venturi type gas mixer and the two upper manifolds receive gases through lines that in clude a second gas mixer. Each of these gas mixers receives air through one inlet and a combustible gas through the other inlet, with the air originating from an air blower mounted to the frame of the oven and passing through an air regulator valve that controls the flow of the air to the mixer in accordance with the pressure within the burners.

Similarly, the gas supply is connected to the other mixer through a gas regulator, valve and this regulator valve controls the flow of gas to the mixer in accordance with the pressure between the blower and the air regulator in the airline during the start-up whereby the low pressure in the airline causes a substantial flow of gas to provide a rich mixture for ignition. After' approximately ten seconds, the pressure from the airline between the blower and the air regulator is disconnected and the gas regulator valve is controlled by the pressure in the burner in the same manner as the air regulator.

A second shut-off valve closes the gas line whenever an infrared sensitive photocell determines that the plastic sheet in the oven is too hot. The gas line is also closed between cycles. When the gas line is closed and the air line is open, the air regulator senses a lowered pressure in the burner and supplies a strong flow of air to cool the heated plastic sheet, if one is present, and to clear the lines of gas.

The above-noted and other features of the invention will be better understood from the following detailed description when considered with reference to the accompanying drawings-in which: 1

FIG. 1 is a partly exploded, perspective view of a thermoforming apparatus that incorporates an embodiment of the invention;

FIG. 2 is a fragmentary perspective view of the oven station in the thermoforming apparatus shown in FIG. 1 when viewed from the right-hand side as shown in FIG.

FIG. 3 is a fragmentary perspective view of the oven station when viewed from the left-hand side as shown in FIG. I;

FIG. 4 is an exploded perspective view, partially schematic, of the oven station; and

FIG. 5 is a schematic diagram of the control system for controlling the operation of the oven station as shown in FIGS. 14.

GENERAL FEATURES OF THE THERMOFORMING APPARATUS In FIG. 1, a thermoforming machine 10 is shown having three operating stations, which are: (l) a loading and unloading station 12; (2) an oven station 14; and

(3) a molding station 16. Each of the operating stations 12, 14, and 16 includes a stationary frame section with a respective one of the three recangular base sections 18, 20 and 22; a respective one of the three roof sections 24, 26 and 28; and a different pair of uprights between the respective base and roof sections shown at 30, 32, and 34 respectively.

At the center of the three operating stations 12, 14 and 16, is a center post and turret assembly 36 having a base portion 38 mounted to the floor and to each of the rectangular base sections 18, 20 and 22 of the operating stations. Extending vertically from the center of the base 38 is a rotatable turret of the center post and turret assembly 36 carrying the rotatable dial plate 40 and being driven by a suitable motor mechanism 41 attached to the base plate 38.

To carry sheets of plastic between the operating stations, three sheet carrying frames 42, 44 and 46, are mounted to and carried by the dial plate 40 at spaced apart'locations around the turret so that each of the operating stations 12, 14 and 16 normally has one sheet carrying frame within it during operation. The sheet carrying frames are rotated by the dial plate 40 through each of the operating stations, with: (l) a molded article being removed from the sheet carrying frame and an unformed sheet being placed in the sheet carrying frame in the operating station 12; (2) a sheet of plastic being heated in the operating station 14; and (3) the sheet of plastic being molded within the molding station 16.

To shape a heated sheet of plastic, the molding station 16 includes two movable platens 48 and 50 which are adapted to carry dies. The dies are movable with the platens in a vertical direction toward the sheet-carrying frame near the center of the molding station 16 to shape the sheet of plastic and away from the center of the molding station to permit the sheet carrying frame to move out of the station and be replaced by another sheet carrying frame with a heated, unformed sheet of plastic in it.

Priorto molding, the sheet of plastic is heated to the proper temperature within the oven station 14.

The oven station 14 includes an upper heating section 52 and a lower heating section 54 (FIGS. 2, 3 and 4). The upper and lower heating sections include a plurality of gas burners for heating the plastic sheet as will be explained in greater detail hereinafter.

In operation, the operator places a plastic sheet of thermoformable material in the carrying frame that is in the loading and unloading station 12, which is shown as carrying frame 42 in FIG. 1, and clamps it therein. The operator then actuates two switches to cause the dial plate 40 to rotate.

The sheet newly placed in the carrying frame 42 of the loading and unloading station 12 is rotated into the oven station 14 between the upper heating section 52 to the molding station 16 for forming by dies mounted on the movable platens 48 and 50. After the plastic sheet has been formed, the sheet carriers again are rotated to bring the molded sheet into the loading and unloading station 12 for removal.

STRUCTURE OF THE OVEN In FIGS. 2, 3 and 4, the upper and lower heater sections 52 and 54 of the oven station 14 are shown in some detail.

As best shown in FIG. 4, a bottom oven section 54 includes a plurality of parallel elongated burners 56A-56E extending across the length of the oven section 54. The upper oven section 52 includes a corresponding plurality of parallel elongated burners 58A-58E.

Each of the burners 56A-56E and SSA-58E comprises an upper chamber indicated in the burner 56D in FIG. 3 at 60 and the lower compartment indicated in the burner 56D at 62. The lower compartment is a gasair mixture feed manifold and the upper compartment is an expansion chamber covered with a fine wire screen indicated at 64' on the burner 56D. Between the chambers 60 and 62 of each of the burners is a meter reading orifice and adjustable mixture distributor through which the gas-air mixture passes from the bottom chamber to the upper chamber where it isbumt. The combustion of the gas is complete so that no vents or exhaust stacks are included in the therrnoforming apparatus.

The radiation from the burners is infrared. Suitable burners for use in the elongated burners 56A-56E and 58A58E may be purchased in sections to be attached together from Pyronics, Inc., 17700 Miles Avenue,

Cleveland, Ohio.

The elongated bumers 58A-58E in the upper oven section 52 have their combustion chamber and screen facing downwardly and the burners 56A and 56E in the heater section 54 have their combustion chambers and screens facing upwardly so that the infrared heat is radiated toward the center of the oven station 14. Near the center of the oven station between the upper oven section 52 and the lower oven section 54, a frame 66 is mounted holding the relatively wide mesh screen 68. The plastic sheet carriers 42, 44 and 46 (FIG. 1) pass above the screen and below the upper oven section 52. The screen 68 serves to prevent the sheet of plastic, which droops upon heating, from contacting the burners 56A-56E.

To supply the air and gas mixture to the burners 56A-56E of the lower section 54, a corresponding one of the inlet pipes 70A-70E (FIG. 3) FIG. with each of the burners 56A-56E and, to supply the mixture of air and gas to the burners SSA-58E of the upper oven section 52, a corresponding one of the plurality of inlet pipes 72A-72E communicates with each of the burners 58A-58E. Each of the inlet pipes 70A-70E communicates with two manifolds 74 and 76 that store a mixture of air and gas and each of the inlet pipes 72A-72E communicates with two manifolds 78 and 80 that likewise store a supply of air and gas. The inlet pipes are each connected to the manifolds in the same manner as illustrated by the inlet pipe 70D (FIG. 3) which is connected to the manifold 76 through the pipe 82D, the valve 84D, and the pipe 86D. The inlet pipe 70D is connected to the manifold'74 through the pipe 88D.

The valve 84D controls the pressure at which the mixture of gas and air flows in the manifold 76 into the inlet pipe 70D. A similar valve is provided for each burner to control the pressure at which the gas is burned in that burner. The pressure at which the gas is burned controls the light spectrum emitted from the burner and the pressure is adjusted to provide the desired frequency spectrum of infrared radiation for the type of plastic to be heated. v

To supply air to the manifolds, a high speed air blower 90 is provided (FIG. 2) mounted to one of the two uprights 32 of the oven station 14. The upright is tubular and the air blower 90 communicates with its interior. A mixture of air and gas is supplied to the manifold 74 in the bottom oven section 54 through the following orifices and conduits in the order named, which are: (l) the lower orifice 92 in the tubular upright 32; (2) a hose 96 shown schematically in FIG. 4; (3) a gas mixer 98,'where the air is mixed with a combustible gas; (4) a T-joint pipe 160; and (5) a hose 100 connecting-one outlet of the T-joint pipe 160 to the manifold 74. To supply air to the manifold 76 of the lower section, a connection is made between the other outlet of the T-joint pipe 160 and the manifold 76 through the conduit 161.

Air is supplied from the interior of the tubular upright 32 to the manifold 78 in the upper oven section 52 through the following conduits and orifices in the order named, which are: (l) the upper orifice 102 in the tubular upright 94; (2) the hose 104; (3) the main air regulator 106: (4) the gas mixer 108, where the air is mixed with a combustible gas; (5) the T-joint pipe 110; and (6) the conduit 112 connecting the T-joint pipe 110 to the manifold 78. To supply air to the manifold 80 in-the upper oven section 52 the other outlet of the T-joint pipe 110 communicates with the manifold 80 through a conduit 1 13.

.To supply combustible gas to the manifolds 74, 76,

78 and so, a gas intake pipe 115 (FIG. 3 and FIG. 4 is provided. The gas intake pipe 115 is connected to a source of propane or natural gas and may be connected to a supply from a public utility.

As best shown in FIG. 4, the source of gas from the gas intake pipe 1 15 is supplied to the manifold 78 in the upper oven section 52 through the series of orifices and conduits that communicate with one another in the order named, which are: (l the main gas shut-off valve 114 which communicates with the gas intake pipe 1 15; (2) a housing 116 for receiving a low gas pressure switch 118; (3) a T-joint pipe 120; (4) a main gas safety valve 122; (5) a housing 124 for the safety check shutoff valve pressure gage 126; (6) a safety check shut-off valve 128; (7) a housing 130 for a high pressure switch 132; (8) a housing 134 for the vent valve 136; (9) a main gas blocking valve 137; a T-joint pipe 138; (l I) a safety check shut-off valve 140; l2) a main gas pressure regulator 142; (13) a conduit 144; (14) the air-gas mixer 108 in which the combustible gas is mixed .with air; a T-joint pipe 110; and (16) a section of conduit 112 connecting one outlet of the T-joint pipe 110 to the manifold 78.

The manifold 76 receives the mixture of gas and air from the other outlet of the T-joint pipe 110.

The mixture of air and gas is applied to the manifold 74 from one outlet of the T-joint pipe through the following conduits connected together in the order named, which are; (l) a valve 148; (2) a T-joint pipe 150; (3) a conduit 152; (4) a valve 154; (5) a gas pressure regulator 156; (6) a conduit 158; (7) the gas mixer 98 in which the gas is mixed with air; (8) a T-joint pipe 160; and, (9) a conduit 100 connecting one of the outlets from the T-joint pipe 160 to the manifold 74.

The manifold 80 receives the mixture of gas and air from the other outlet of the T-joint pipe 160.

To supply a source of gas for the pilot lights, a conduit 162 communicates with one of the outlets of the T joint pipe 138 and with a pressure regulator 164 through a valve 166. The pilot light is supplied with gas through the conduit 168 from pressure regulator 164. Air is supplied to the pilot lights through a similar T- joint pipe in the air line. There is a separate pilot burner for the upper oven section 52 and for the lower oven section 54. v y

A portion of the combustible gas is supplied to the burners at a fixed rate during their operation. For this purpose, each of the burners 56A-56E of the lower oven section 54 communicates with the manifold 74 through a different one of the conduits l70A-l70E and each of the burners SSA-58E of the upper oven section 52 communicates with the manifold '80 through a different one of the conduits 172A-172E.

To control the rate of flow of gas and the gas pressure to an individual one of the rows of burners 56A-56E of the lower oven section 54, each of the rows of burners 56A-56E communicates with the manifold 76 through a different one of the conduits 174A-l74E, which conduits each include a different one of the valves l76A-l76E. Similarly, the rate of flow of gas and the gas pressure to the burners SSA-58E is controlled by a corresponding one of the valves A-180E in its respective conduit, l78A-178B, which communicates with the burner and manifold.

THE OVEN CONTROL SYSTEM i In FIG. 5, a simplified schematic diagram is shown of the main control elements for the flow of combustible gas and air to the manifolds. The diagram omits some control units for the purpose of simplifying the explanation and adds other units that are not visible in the other drawings. Although only a supply of gas for the manifold 78 is illustrated, a similar control arrangement is incorporated for each of the other three manifolds. I

As shown in FIG. 5, the air regulator 106 for the air supply is controlled by a regulator control device 182 and the gas regulator 142 for the combustible gas supply is controlled by a regulator control device 184. The air regulator 106 and the gas regulator 142 are equipped with regulator control devices that control the regulators in response to the fluid pressure applied to the regulator control device. Gas regulators incorporating such control devices may be purchased from Pyronics, Inc., 17700 Miles Avenue, Cleveland, Ohio.

The fluid pressure permitted by the regulators are controlled by the regulator control devices 182 and 184 in response to changes in the pressure of the gas lines to which they are connected. For this purpose, the

regulator control device 182 communicates with the gas between the gas mixer 108 and the manifold 78 through the conduit 186 and causes the air regulator 106 to open to permit more air to pass to the gas mixer 108 when the pressure between the gas mixer and the manifold 78 decreases. Similarly, in normal operation, the regulator control device 184 communicates with the gas between the gas mixer 108 and the manifold 78 through the conduit 188 to open the gas regulator 142 and to permit more gas to pass to the gas mixer 108 when the pressure in the line between the gas mixer 108 and the manifold 78 decreases.

To provide a richer gas mixture to the burners during the ignition of the burners, a cycle starting signal 190 starts a timer 192 to connect the regulator control device 184 to the air line between the blower 90 and the air regulator 106 through a conduit 194 at the beginning of a cycle. Ten seconds later, the timer 192 causes the regulator control device 184 to be disconnected from the conduit 194 and to communicate with the conduit 188 so as to provide a normal mixture of combustible gas and air.

A pilot burner 196 communicates with the blower 90 through a conduit 198 and with the natural gas source 206 through a conduit 200 to ignite the burners. A separate pilot burner is provided for the upper and lower oven sections. With this arrangement, the burners are extinguished between cycles and ignited by the pilot burner 196 at the start of each cycle.

To prevent the sheet of plastic held in the oven station from being overheated, photocell sensing devices, one of which .is shown at 202, are provided in the oven station and respond in proportion to the temperature of the plastic because of the intensity of infrared radiation therefrom. The photocell 202 is electrically connected to the electrically controlled shut-off valve 204 which causes the natural gas source 206 to be closed to the gas regulator 142 when the temperature of the sheet of plastic rises about a predetermined limit.

When the gas regulator 142 is closed to natural gas, the burners are extinguished and the pressure between the gas mixer 108 and the manifold 78 decreases, caus- 'ing the regulator control device 182 to open the air regulator 106 and to increase the flow of air to the gas mixer 108. This increase in the flow of air causes air to be blown on the sheet of plastic to cool it.

OPERATION In operation, a sheet of plastic is clamped in the sheet carrying frame 42 within the loading and unloading sta-.

where it is held above the screen 68 (FIGS. 2, 3 and 4).

With the sheet of plastic in the oven station 14, a

cycle of operation is started. This cycle will be described, in part, by reference to FIG. 5 which illustrates the control system for the manifold 78, but the operation is the same for all manifolds.

When the cycle is started, the cycle starting signal 190 (FIG. 5) is applied to the timer 192, (one of which is shown in FIG. 5) causing the regulator control device 184 to be connected to the air line between the blower 90 and the air regulator 106.

With this connection, a low pressure causes the gas regulator 142 to open widely. Natural gas flows from the natural gas source 206 through the gas inlet pipe 115 and the electrically controlled shut-off valve through the gas regulator 142 which is opened widely to pass the combustible gas into the gas mixer 108. The blower supplies air through the air regulator 106 to the gas mixer 108 so that a rich mixture of combustible gas and air is supplied to the manifolds. The regulator control device 182 is connected by the conduit 186 to the location between the gas mixer 108 and the manifold to cause the air regulator 106 to pass the proper amount of air to the gas mixer 108.

The rich mixture of gas is ignited by the pilot burner 196 which receives air from the blower 90 through the line 198 and combustible gas from the natural gas source 206 through the lines 1 15 and 200.

After ten seconds, the timer 192 causes the regulator control device 184 to be connected through the conduit 188 to the gas line between the gas mixer 108 and the manifold and disconnects the regulator control device 184 from the line 194. With this connection, the gas regulator 142 is controlled by the pressure between the gas mixer 108 and themanifold in the same manner as the air regulator 106 to provide a normal operating flow of gas to the manifold.

The burners 56A-56E in the lower oven section 54 each receive a mixture of gas and air from the manifold 76 through corresponding conduits and valves 174A-174E and 176A-176E, respectively, and from the manifold 74 through the conduits 170A-170D. The

burners SSA-58E each receive combustible gas from the manifold 78 through corresponding conduits 178A-178E and valves l80A-180E and from the manifold 80 through the conduits 172A-172E.

The rate of flow of gas through the system is controlled near the intake hose by the manual valve 114. It is automatically regulated just beyond the manual valve by the low gas pressure regulator 118.

Beyond the low gas pressure regulator 118, the gas that is to be applied to the upper manifolds passes through one branch of the T-joint pipe whereas the gas to be applied to the lower manifolds passes through the other branch.

The gas that is to be applied to the upper manifolds is regulated by the main gas safety valve 124 and the safety check shut-off valve 126. It may also be manually shut off at the hand valve 128.

Beyond the manual safety check shut-off valve 128, the flow of gas is automatically regulated by the high pressure regulator 132 and is passed to the T-joint pipe 134 which is connected to the vent valve 136. After this, the gas is controlled by the main gas blocking valve 137 and passed to the T-joint pipe 138 where one portion of the gas flows through the conduit 162, the valve 166, the pressure regulator 164, and the conduit 168 to the pilot burner 196 (FIG. 5) and another portion flows through the other joint of the T-joint pipe 138 to. the safety check shut-off valve 140. From the safety check shut-off valve 140 the combustible gas flows through the main gas regulator 142 and into the gas mixer 108 where it is mixed with air and passed to the T-joint pipe 110. Each of the two outlets of the T-v joint pipe 1 10 communicates with a different one of the two upper manifolds 78 and 80.

The combustible gas for the lower manifolds 74 and 76 flows through the other outlet of the T joint pipe 120 through the valve 148 and the T-joint pipe 150. The T-joint pipe 150 supplies gas to the pilot light 196 for the upper oven section 152 and supplies gas to the upper and lower manifolds through the valve 154, the automatic pressure regulator 156, the conduit 158, the gas mixer 98, where the combustible gas is mixed with air, and the T-joint pipe 160 which communicates with the two lower manifolds 74 and 76.

To supply air to the four manifolds, a blower 90 is mounted to one of the hollow tubular uprights 32 in the oven station 14 and supplies air under pressure to the interior of the upright. A hose 96 communicates with the lower orifice 92 in the tubular upright and supplies air to one of the inlets of .the gas mixer 98 for mixing with the combustible gas. The mixture is applied to the T-joint pipe 160 and from the T-joint pipe 160 to the lower manifolds 74 and 76. Air flows through an upper orifice 102, a hose 104, and an air regulator 106 to the gas mixer 108 where it is mixed with a combustible gas. The mixture is then applied to the T-joint pipe 110 from which it is applied to the upper manifolds 78 and 80.

The gas mixture for the lower manifolds 74 and 76 is adjusted by the valve 154 and the gas mixture in the manifolds 78 and 80 is adjusted by the valve 170 to control the gas supply to the mixing valves 98 and 108. The gas pressure for each of the heaters 56A-56E is individually adjusted by a corresponding one of the valves l76A-l76E which controls the flow from the manifold 76 and the gas pressure in each of the burners SSA-58E is individually adjusted by the respective one of the valves l80A-180E to provide a wide range of control over the pressure in the burners. It is not necessary to individually control the rate of flow of gas and the pressure from both of the manifolds to the burners since a sufficient range of control is provided by controlling the gas flow from one of the manifolds.

As the sheet of plastic in the oven section is heated, it is continually monitored by the infrared sensitive photocell 202. Should the sheet of plastic become too hot, the electrically controlled'shut-off valve 204 is activated by the photocell 202 to close the gas line 115. With the gas line 115 closed, the How of gas between the mixing valve and the manifolds is reduced causing the regulator control device (shown as 182 in FIG. to sense a lower pressure and open the air regulator to supply more air to the manifolds. With the gas disconnected, there is no further flame and air is blown on the plastic to cool it.

In the normal operation of heating a sheet of plastic, the gas burners supply infrared heat that is radiated upon the sheet of plastic without being substantially absorbed by the atmosphere. The spectrum of the infrared radiation is controlled by the gas pressure set by the hand valves in the gas flow system. It is selected to be suitable for the thickness of the plastic sheet and the type of plastic to provide the most efficient heating.

The burners provide full heat output within one minute rather than the 20 or 30 minutes required for conventional electrical ovens. The rapid start-up time increases the speed of operation of the thermoforming machine and prevents the clamp frame cylinders from being over-heated due to continuous running time. The

10 rapid shutdown of the oven also reduces the possibility of fire caused by over-heated plastic sheets. Moreover, the use of gas heat reduces the cost of supplying heat.

After the plastic sheet is sufficiently heated, it is rotated to the forming station 16 where dies mounted to the platens 48 and are brought into contact with the sheet to form it. The sheet is then rotated again to the loading and unloading station, where it is removed and a new sheet inserted.

Although a specific embodiment has been described with some particularity, many variations are possible in the preferred embodiment without deviating from the invention. it is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. An oven for a thermoforming apparatus comprisa plurality of fixed, horizontal rows of gas-fired, in-

frared radiating burners with a plurality of said burners being fixedly positioned in each of said rows,

means for supporting a sheet of plastic in proximity to said burners in a horizontal position for the heating thereof, and

control means for independently controlling the flow of air and combustible gases to said rows of burners and for independently controlling the mixture of air and combustible gases supplied to each burner in each of said rows,

whereby the infrared radiation impinging on the portions of said plastic sheet disposed opposite to each of said burners is separately controllable.

2. An oven according to claim 1 in which said control means includes a means for providing a heavier flow of gas at the time of ignition of the burners than during normal use of the burners.

3. An oven according to claim 1 in which said control means includes a means for applying air to a plastic sheet supported within the oven through said burners, whereby said article is cooled.

4. An oven according to claim 3 in which said means for applying air includes a means for terminating the flow of combustible gas to the burner while permitting the flow of air to the burner.

5. An oven according to claim 4 in which said means for terminating the flow of combustible gas while permitting the flow of air includes a sensing means to terminate the flow of combustible gas when the temperature of said sheet exceeds a predetermined value.

6. An oven according to claim 1 in which said control means comprises:

a first gas regulator adapted to be connected to a source of air;

a second gas regulator adapted to be connected to a source of combustible gases;

a mixing valve having one input communicating with said first gas regulator and the other input communicating with a second gas regulator;

a manifold;

said manifold communicating with the outlet of said mixing valve;

conduit means connecting said manifold to said rows of burners; and

gas pressure sensing means for controlling said first and second gas regulators in accordance with pressure changes within said heaters.

7. An oven according to claim 6 further comprising:

a shut-off valve communicating with the gas flow from said source of combustible gas;

an infrared sensing device positioned within said oven to sense the temperature of an item therein; and

means for closing said shut-off valve when said photocell senses a temperature above a predetermined limit.

8. An oven according to claim 6 in which said gas pressure sensing means includes:

means, having an inlet for controlling said second gas regulator in accordance with gas pressure at its inlet;

said oven having a cycle with a starting period for igniting the gas burners; and

timer means for connecting said inlet of said means for controlling said second gas regulator to said source of air for a short period of time at the beginning of a cycle and to the gas in said heaters for a period of time thereafter.

9. An oven according to claim 6 in which said control means further comprises:

a second manifold;

second conduit means for connecting said second manifold to said heaters;

said second conduit means including first manual valve means for adjusting the rate of flow of gases through said conduit means;

said manifold communicating with said outlet of said mixing valve;

second manual valve means positioned to regulate the flow of air manually to said mixing valve; and

third manual valve means for regulating the flow of combustible gases to said mixing valve.

10. An oven according to claim 1, in which each burner includes a first compartment for receiving air and combustible gases and a second compartment for burning combustible gases, and in which said control means includes means connecting said first and second compartments for controlling the flow of air and combustible gases between said first and second compartments. 

1. An oven for a thermoforming apparatus comprising, a plurality of fixed, horizontal rows of gas-fired, infrared radiating burners with a plurality of said burners being fixedly positioned in each of said rows, means for supporting a sheet of plastic in proximity to said burners in a horizontal position for the heating thereof, and control means for independently controlling the flow of air and combustible gases to said rows of burners and for independently controlling the mixture of air and combustible gases supplied to each burner in each of said rows, whereby the infrared radiation impinging on the portions of said plastic sheet disposed opposite to each of said burners is separately controllable.
 2. An oven according to claim 1 in which said control means includes a means for providing a heavier flow of gas at the time of ignition of the burners than during normal use of the burners.
 3. An oven according to claim 1 in which said control means includes a means for applying air to a plastic sheet supported within the oven through said burners, whereby said article is cooled.
 4. An oven according to claim 3 in which said means for applying air includes a means for terminating the flow of combustible gas to the burner while permitting the flow of air to the burner.
 5. An oven according to claim 4 in which said means for terminating the flow of combustible gas while permitting the flow of air includes a sensing means to terminate the flow of combustible gas when the temperature of said sheet exceeds a predetermined value.
 6. An oven according to claim 1 in which said control means comprises: a first gas regulator adapted to be connected to a source of air; a second gas regulator adapted to be connected to a source of combustible gases; a mixing valve having one input communicating with said first gas regulator and the other input communicating with a second gas regulator; a manifold; said manifold communicating with the outlet of said mixing valve; conduit means connecting said manifold to said rows of burners; and gas pressure sensing means for controlling said first and second gas regulators in accordance with pressure changes within said heaters.
 7. An oven according to claim 6 further comprising: a shut-off valve communicating with the gas flow from said source of combustible gas; an infrared sensing device positioned within said oven to sense the temperature of an item therein; and means for closing said shut-off valve when said photocell senses a temperature above a predetermined limit.
 8. An oven according to claim 6 in which said gas pressure sensing means includes: means, having an inlet for controlling said second gas regulator in accordance with gas pressure at its inlet; said oven having a cycle with a starting period for igniting the gas burners; and timer means for connecting said inlet of said means for controlling said second gas regulator to said source of air for a short period of time at the beginning of a cycle and to the gas in said heaters for a period of time thereafter.
 9. An oven according to claim 6 in which said control means further comprises: a second manifold; second conduit means for connecting said second manifold to said heaters; said second conduit means including first manual valve means for adjusting the rate of flow of gases through said conduit means; said manifold communicating with said outlet of said mixing valve; second manual valve means positioned to regulate the flow of air manually to said mixing valVe; and third manual valve means for regulating the flow of combustible gases to said mixing valve.
 10. An oven according to claim 1, in which each burner includes a first compartment for receiving air and combustible gases and a second compartment for burning combustible gases, and in which said control means includes means connecting said first and second compartments for controlling the flow of air and combustible gases between said first and second compartments. 